1. Field of the Invention
This invention relates to a coating apparatus of the extrusion type called a die coater.
2. Description of the Prior Art
Hitherto, as a method of applying a magnetic coating composition in manufacturing a magnetic recording medium of the coating type, the roll coating system using a roll as represented by the gravure roll system or the reverse roll system, etc. has been a main current.
However, in such a roll coating system, there are the problems of a change in the output due to unevenness of the coating thickness by an unsatisfactory coating transfer from the roll to the base film, drop out by the coating splash, and degradation in the coating quality as the result of the fact that the excess or surplus supplied coating liquid or solution is returned to the mixing process. Particularly, as the coating speed become high, such problems have a tendency to become remarkable.
In recent years, as a new coating system capable of solving these problems, attention has been drawn to the extrusion (die) system. In the process for manufacturing coating type magnetic recording media, such an extrusion system has been already put to practice in part.
The extrusion system uses a die which has a narrow slit of a predetermined width at the front end portion thereof and includes a doctor edged surface in the vicinity of the front end portion of the die, to coat a coating liquid, onto a moving support, and the coating liquid is continuously extruded toward the surface of the moving support and is spread by the doctor edged surface with a uniform thickness. Such an extrusion system has been used until now in the photographic field such as for photographic film or photographic (printing) paper, etc.
Further, if an attempt is made to manufacture a so called magnetic recording medium of the double layer type by using, e.g., the roll coating system, the same process step is required to be repeated twice. As a result, the process becomes complicated, giving rise to inconveniences such that the manufacturing cost is increased. In addition, with the above-mentioned roll coating system, it is difficult to carry out a thin coating such that the thickness of the upper layer is less than 1 .mu.m. For this reason, it is the present circumstances that this roll coating system is hardly applied in the manufacturing of a video tape, etc.
Meanwhile, in the coating apparatus of the extrusion system, it is known that the shape of the front edge surface of the die in contact with the flexible support has a great influence on the coating film quality, and improvements thereof have been developed in various fields until now.
For example, in U.S. Pat. No. 4,424,762, U.S. Pat. No. 4,480,583, U.S. Pat. No, 4,681,062, U.S. Pat. No. 5,042,422, and the Japanese Patent Application Laid Open No. 35959/1990, etc., there are disclosed coating apparatuses in which the shape of the die edge portion (so called a lip portion) is prescribed in various manners.
However, even with these technologies, it is impossible to sufficiently suppress an occurrence of coating unevenness or streak-like irregularity, etc. For this reason, further improvement is expected.
Further, as the technology for carrying out double layer coating in the above-mentioned coating apparatuses of the extrusion system, the following technologies are generally enumerated.
A first method is a method as disclosed in the Japanese Patent Application Laid Open No. 261562/1990 or the Japanese Patent Application Laid Open No. 268,862/1990, in which different coating liquids are simultaneously fed from two pockets provided in the die into a single slit. A second method is a method as disclosed in the U.S. Pat. No. 4,854,262 or the Japanese Patent Application Laid Open No. 251265/1990, in which two slits are provided in the die to extrude coating liquid from these two slits onto a support continuously running in sequence along the front edge surface, the center edge surface, and the back edge surface. A third method is a method as disclosed in the Japanese Patent Application Laid Open No. 153/1991 publication, etc., in which two dies formed entirely in separate bodies are used to separately apply coating onto respective layers.
However, even in the case where any methods described above are employed, it is difficult to carry out double layer simultaneous coating having less unevenness of the coating thickness. For example, in the case of the first technology, two kinds of coating liquids flow in a manner of two-liquid laminar flow into a single slit. As a result, the smooth boundary surface between coated films of the two layers obtained as the result of the fact that two kinds of coating liquids are mixed at the boundary is difficult to obtain. Thus, there occurs unevenness in the coating thickness in both layers. Also, in the case of the second technology, since a coating layer coated by the first slit shifts to the next slit in the liquid state in contact with the center edge surface, the next layer is coated on the first layer as it remains in the liquid state. As a result, similarly to the previously mentioned first technology, two kinds of coating liquids are mixed at the boundary thereof, so there occurs unevenness of the coating thickness in the both layers. In addition, in the case of the third technology, the distance from coating of the lower layer up to coating of the upper layer is increased. As a result, the balance of solvent with the upper layer is lost resulting from drying of the lower layer so that the subsequent coating is difficult to carry out. Thus, there also occurs unevenness in the coating thickness.